1. Field of the Invention
The present invention relates to a lactic acid-based polymer resin composition and molded articles thereof.
More specifically, the invention relates to a lactic acid-based polymer resin composition and molded articles thereof which have excellent thermal resistance, impact resistance and molding ability and can degrade after use in the natural environment.
2. Description of the Related Art
Generally, containers having excellent thermal resistance and impact resistance are prepared from polypropylene, crystalline polyethylene terephthalate (hereinafter referred to as PET) and other resins.
However, these resins increase the amount of refuse in the waste disposal after use. Additionally, these resins are scarcely degraded in the natural environment, and thus semipermanently remain in the ground even after burial disposal. Scenery is also impaired by disposed plastics and problems of destroying living environment of marine organisms have emerged.
On the other hand, polylactic acid, a copolymer of lactic acid with other aliphatic hydroxycarboxylic acids and polyesters derived from aliphatic polyhydric alcohol and aliphatic polycarboxylic acid have been developed as thermoplastic polymers having biodegradability.
These polymers can completely biodegrade in an animal body within several months to a year. When placed in soil or sea water, these polymers start degradation within several weeks under wet conditions and disappear in one to several years. Further those polymers are characterized by finally decomposing into lactic acid, carbon dioxide and water which are nontoxic to the human body.
Particularly, polylactic acid is expected to extend the field of its use, because the raw material L-lactic acid has recently come to be manufactured in a large scale and cheaply by a fermentation process and polylactic acid has an excellent characteristic of high stiffness.
However, containers and packaging materials obtained by common injection molding and other processing methods have a disadvantage of thermal resistance, though excellent in stiffness. Consequently, polylactic acid was inadequate for uses in high temperature, for example, a container wherein hot water is poured and a container for use in a microwave oven.
The prior art in order to provide thermal resistance for polylactic acid differs from common processing technique, and specific examples thereof include, for example, a technique for not taking the molded article out of the mold immediately after processing, but holding the article as intact in the mold for a long time at a temperature around TC (crystallization temperature) until the polylactic acid molded article obtains high crystallinity, and a technique for processing by conventional techniques and successively carrying out annealing (heat treatment) of the molded article to give high crystallinity.
However, these conventional processing techniques for providing polylactic acid with thermal resistance, for example, crystallization is not always satisfactory in the former case and the molded article sometimes deforms in the course of crystallization in the latter case. Thus each of these processes has problems, respectively.
Further, these conventional processing techniques for providing thermal resistance for polylactic acid require specific conditions or very long time in the steps as compared with common processing techniques. As a result, these techniques increase manufacturing cost and are not always practical.
Specific examples of techniques for accelerating the crystallization velocity of general purpose high polymer materials include, for example, the technique disclosed in Japanese Laid-Open Patent Sho 60-86156.
The patent has disclosed a technique for adding fine powder of overall aromatic polyester having primary constitution units of terephthalic acid and resorcinol as a nucleating agent (crystallization accelerator) in order to accelerate crystallization of PET.
Techniques for providing thermal resistance for degradable high polymer materials have been known to involve adding a nucleating agent similar to the case of the general purpose high polymer materials. Specific examples which have disclosed these techniques include, for example, WO 90/01521, U.S. Pat. No. 5,180,765, WO 92/04413 and Japanese Laid-Open Patent Hei 4-220456.
WO 90/01521 has disclosed a technique for increasing hardness, strength and thermal resistance by addition of inorganic fillers such as silica and kaolinite to lactide based thermoplastic resins. An example thereof involves preparing a sheet having increased crystallinity by adding 5% by weight of calcium lactate as a nucleating agent to an L- and DL-lactide copolymer and blending on hot rolls at 170 .degree. C. for 5 minutes. The high-crystalline sheet is described to be excellent in stiffness and strength, but low in transparency and cloudy.
The present inventors practically applied the technique to polylactic acid. That is, injection molding was tried by adding silica, kaolinite and talc as nucleating agents to polylactic acid. However, problems were found at least in the following two points and a molded article suited for practical use could not be obtained.
1) Slow crystallization velocity and insufficient crystallinity
2) Deterioration of high polymer component and embrittlement of molded article
WO 92/04413 has disclosed a technique for adding lactic acid salt or benzoic acid salt as a nucleating agent to degradable polymers. In the examples of the invention, it has been described to blend a polylactide copolymer with 1% of calcium lactate, carry out injection molding with a residence time of 2 minutes in the mold by using a mold maintained at around 85.degree. C., and thereafter anneal in the mold at 110-135.degree. C.
The present inventors practically applied the technique to polylactic acid. That is, injection molding was tried by adding calcium lactate and sodium benzoate as nucleating agents to polylactic acid. However, problems were found at least in the following two points and a molded article suited for practical use could not be obtained.
1) Slow crystallization velocity and insufficient crystallinity
2) Deterioration of high polymer component and embrittlement of molded article
Japanese Laid-Open Patent Hei 4-220456 has disclosed a technique for adding polyglycolic acid and a derivative thereof as a nucleating agent to poly-L-lactide and increasing mold temperature to Tg (glass transition temperature) or higher in the injection molding step. By the technique, crystallization velocity is increased, molding cycle time is reduced, and the molded product obtains excellent mechanical properties. It is described in the example of injection molding that crystallinity is 22.6% after cooling for 60 seconds in the absence of a nucleating agent, and 45.5% in the presence of the nucleating agent.
The present inventors practically applied the technique to polylactic acid. That is, polyglycolic acid was added to polylactic acid as a nucleating agent, mold temperature was increased to Tg or higher, and injection molding was tried. However, injection molding was failed when the mold temperature was Tg or higher.
The present inventors have already disclosed in European Patent A-661346 a technique for mixing polylactic acid with crystalline inorganic powder which contains 50% or more of crystalline SiO.sub.2 as a nucleating agent, melting the resulting mixture and carrying out molding while proceeding crystallization. The technique can provide heat resistant molded articles and is thus very significant. However, the technique requires filling molten resin in a mold which is maintained at 85-125.degree. C. in order to crystallize the resin in the mold, and thus has the following problems.
1) A mold having special structure and oil circulating equipment for maintaining the mold at high and constant temperature are required and result in high cost of plant and equipment.
2) Time is required for crystallizing in the mold and thus a molding cycle is prolonged and productivity becomes poor as compared with molding of a general purpose resin.
3) A mold for molding complicated articles or a multiple cavity mold is difficult to design due to special structure of the mold.
4) The above disadvantages lead to high manufacturing cost.
As mentioned above, in the case of providing thermal resistance for polylactic acid, simple application of a common nucleating agent such as talc, silica and other inorganic materials, organic carboxylate salt and polymer to a conventional processing method such as injection molding, blow forming and compression molding cannot provide molded articles which have high crystallinity and thermal resistance to temperatures of 100.degree. C. or more.